Traditional speech networks for telephone sets have been constructed with a hybrid transformer, designed to provide some degree of isolation between signals that are simultaneously transmitted and received over a two-wire telephone line. The elimination of the hybrid transformer by using an electronic speech network results in a considerable size and weight reduction, while providing the opportunity for improvements in telephone system performance.
In a typical long-loop application, a minimum line current of 25mA must be drawn from the 48 volt central office batteries to operate the line relays therein. In such an application, the I-R drop along the telephone line limits the d-c voltage drop across the telephone set to 5 volts; and hence its d-c terminal resistance to about 200 ohms. However, since electronic networks, particularly those constructed in integrated circuit form, are susceptible to voltage surges on the line such as caused by lightning strikes etc., surge protection must be provided. This generally necessitates the use of a Zener diode protection circuit including a resistor in series with the line of the order of 47 ohms. In addition, a bridge connected polarity guard must also be connected in series with the electronic network to protect it against polarity reversals from the central office. With about a 1.2 volt drop across the 47 ohm resistor, and another 1 volt drop across the polarity guard, the actual d-c voltage available across the input to the electronic network may well be limited to about 2.8 volts. If the telephone set is to provide a 4 volt peak-to-peak signal (2 volts zero-to-peak) then the network itself must be capable of operating down to 2.8 - 2 = 0.8 volts without clipping the speech signal. However, a typical bipolar transistor has a voltage drop of about 0.6 volts between its base-emitter junction when operating. Consequently this places severe limitations on a network which must operate down to 0.8 volts.
Various such electronic speech networks have been proposed in the past, some utilizing the gyrator principle while others are based on a bridge configuration, to provide the necessary anti-sidetone balance for the telephone set. One such circuit based on this latter concept is disclosed in U.S. Pat. No. 3,440,367 entitled "Non-Reactive Anti-Sidetone Network for a Telephone Set" invented by R. E. Holtz, issued June 2, 1970. Another such circuit is disclosed in U.S. Pat. No. 3,823,272 entitled "Electronic Telephone Transmission Circuit" invented by C. M. Tabalba issued July 9, 1974; while still another is disclosed in applicant's copending United States application Ser. No. 649,557 entitled "Telephone Speech Network" filed Jan. 15, 1976, also invented by G. Spencer now U.S. Pat. No. 4,031,331.
In these electronic hybrid networks of the prior art, a resistor is connected in series with the output of the transmit amplifier across the telephone line, in order to balance the bridge configuration. However, this results in a portion of the transmit signal appearing across this resistor, which is out-of-phase with that across the telephone line. For instance, referring to the attached FIG. 1 (which illustrates a functional circuit diagram of a bridge configuration that is typical of those shown in Holtz and uses the same reference characters as Tabalba) a portion of the transmit signal V.sub.S generated by TR1 is dropped across resistor R5 and the balance across the telephone line impedance ZL. In this bridge arrangement, the transmit signal developed across resistor R5 provides an out-of-phase source which is coupled through resistor R4 and effectively cancels the transmit signal across the input to the receiver coupled from the telephone line through ZB. However, because of the signal drop across resistor R5, the output of the transmitter TR1 must be capable of handling a considerably larger transmit signal without clipping, thus requiring a larger d-c operating voltage across the line terminals of the set. Since the supply voltage from the central office is fixed (nominally 48 volts d-c) the requirement for a higher d-c terminal voltage across the telephone set restricts the loop length. In addition, when such an electronic telephone is placed in parallel with a conventional hybrid transformer telephone in a long-loop application, the heavy current drain of the latter reduces the available voltage on the line to the point where the electronic network may cease to function altogether. Consequently sets employing such electronic networks have generally not met all the operating requirements to enable them to work in parallel with conventional sets on long-loop applications.